This invention relates to a device for inspecting the external view of a substrate on which fillets are formed in a soldering process by obtaining its image while illuminating it from specified directions and using a regularly reflected optical image of the fillets in the generated image to automatically inspect the shape of each fillet. In particular, this invention relates to the technology of setting reference data necessary for the inspection of the fillets to such an inspection device.
Inspection devices of the type for automatically inspecting the external view of a substrate are usually provided with image taking means such as a camera and a line sensor and a controller incorporating a computer. The controller is adapted to take in an image of the target substrate to be inspected by the image taking means, to measure the positions and sizes of target portions to be inspected such as fillets and to judge whether the target portion is good (OK) or not good (NG) by comparing the obtained measured values with specified reference values for judgment.
The assignee herein is currently developing an automatic inspection device incorporating an optical system referred to as the Color Highlight system. As disclosed in Japanese Patent Publication Tokko 6-1173, the Color Highlight system is adapted to irradiate a substrate with red, green and blue light respectively from different angles to obtain an image representing the slope condition of a target portion for inspection (generally a soldered portion) by the distribution of the red, green and blue colors. An inspection device incorporating this Color Highlight system is adapted to judge whether fillets are normally formed or not by detecting areas of each color by means of a binarization process, measuring their positions and sizes and comparing them with reference value for judgment.
When such an inspection device is used to carry out an automatic inspection, various reference data for the inspection must be set and registered in the memory of the inspection device. In the above, the reference data are those that serve to indicate what method should be used how in what steps in order to inspect fillets that are the target portions to be inspected. They may be interpreted as data that represent various standards which must be referenced as a series of processes (such as generation of an image, extraction of target portions to be inspected, measurements and judgments) to be carried out for the inspection.
The aforementioned reference data include, for example, data required for setting a target area of inspection (the position and the size of the area), the kinds of process to be carried out for detecting a target portion to be inspected (such as binarization process, edge extraction process and projection process), methods of the measurement process for a target area for the inspection, and judgment reference value for determining whether the result of measurements was good or no good. If the detection of a target portion to be inspected is carried out by binarization, the threshold value for this binarization process is also included in the reference data.
According to the prior art process for setting reference data for inspection (or the so-called teaching process), an image is obtained of a model substrate with each target portion to be inspected in a good condition (hereinafter referred to as the “good model”) and is displayed, and an inspector sets necessary reference data for each component while observing this displayed image.
It has also been known to preliminarily register standard reference data (so-called library data) for each component in order to reduce the workload for the setting of such reference data for inspection and to read out such library data to be set. Japanese Patent Publication Tokkai 2004-71781 discloses, for example, a process of generating reference data for inspection (referred to as the inspection data) by combining position data of a component read out from CAD data with library data.
According to Japanese Patent Publication Tokkai 11-311508, a specific threshold value for judgment is registered as library data for each component but it is also possible to register rules (referred to as the method of calculating inspection parameters) for obtaining a threshold value instead and to calculate a specific threshold value at the time of the teaching process by using the data on the shape of the component actually mounted to the substrate and the library data. In this way, since it becomes possible to register reference data for inspection common to all components of a same type, it becomes easier to change the rule for setting threshold value and to add new components with different sizes.
Since the shape of a fillet sometimes changes, depending on the height of the cream solder printed on the land prior to the soldering process, it may be difficult to maintain a high level of accuracy in the inspection if same reference data are used uniformly.
One of principal causes of fluctuations in the height of cream solder is in the structure (more particularly the thickness) of the metallic mask that is used in the solder printing process. Since the mask is designed according to the structure of the substrate, the height of the solder may vary even for the same component, depending on the substrate on which it is mounted.
Moreover, since fillets are formed between electrodes on the side of the component and lands, differences in the relative height of the solder with respect to the height of the component may affect the shape of the fillet in the case where the fluctuation in the size of component is large.
Consider a situation where two square chips with different component heights are mounted to a same substrate, and let us assume that the lands for mounting these components are the same in size and shape and that the shape and size of the opening of the masks used for the printing of cream solder are set identically for these lands. If this mask is used to print the cream solder on each land, these two square chips are thereafter mounted and the reflow process is carried out, the melted solder may rise nearly to the height of the upper surface of the electrode on the side of the component and a steep fillet will be formed by the higher square chip while the fillet formed by the lower square chip is sloped more gently.